The basic principle, test equipment and test condition of the capacity-voltage method for measuring the impurity distribution in junction diodes are presented completely. The formula for calculating the effective impurity concentration in the abrupt junction, the linearly graded junction and the random junction is deduced by using the capacity characteristics obtained when the reverse bias are applied to the p-n junction. Finally, the impurity concentration distribution in an IN5401 diode is calculated using the method and the characteristics of the p-n junction are analyzed.